Vapour can be withdrawn from an evaporator by absorption (Figure 2.11) into a liquid. Two combinations are in use, the absorption of ammonia gas into water and the absorption of water vapour into lithium bromide. The latter is non-toxic and so may be used for airconditioning. The use of water as the refrigerant in this combination restricts it to systems above its freezing point. Refrigerant vapour from the evaporator is drawn into the absorber by the liquid absorbant, which is sprayed into the chamber. The resulting solution (or liquor) is then pumped up to condenser pressure and the vapour is driven off in the generator by direct heating. The high-pressure refrigerant gas given off can then be condensed in the usual way and passed back through the expansion valve into the evaporator. Weak liquor from the generator is passed through another pressure reducing valve to the absorber. Overall thermal efficiency is improved by a heat exchanger between the two liquor paths and a suction-toliquid heat exchanger for the refrigerant. Power to the liquor pump will usually be electric, but the heat energy to the generator may be any form of low-grade energy such as oil, gas, hot water or steam. (Solar radiation can also be used.) The overall energy used is greater than with the compression cycle, so the COP (coefficient of performance) is lower. Typical figures are as shown in Table 2.2.
The absorption system can be used to advantage where there is a cheap source of low-grade heat or where there are severe limits to the electrical power available. A modified system of the ammonia–water absorption cycle has been developed for small domestic refrigerators.